1. Field of the Invention
Wafers which are widely used as substrate crystals for electrical integrated circuits are typically produced by slicing a silicon or germanium crystal ingot, and the surface of these wafers is polished to form a flat surface having as little roughness as possible, which are then used for the formation of integrated circuits (IC). This is because, if there are irregularities on the surface of the wafer, precise and fine depiction of the circuit patterns will become difficult, and undesirable electrical characteristics of the IC will result.
Polishing of wafers, especially that of a silicon wafer, which is particularly of industrial importance because of its frequent use, is normally carried out in two steps, primary polishing and final polishing. The primary polishing is a polishing process to remove surface defects formed before the primary polishing process. Since these wafers, before the primary polishing, have defects of approximately 10 to 50 .mu.m, an abrasive is often used to provide a high polishing speed. However, surface roughness of wafers obtained by the primary polishing process is generally 5 m.mu. to 500 m.mu., which is too rough to provide satisfactory performance as substrates for IC's.
Then, final polishing is required. In the final polishing process, wafer surfaces having 5 m.mu. to 100 m.mu. of roughness obtained in the primary polishing process are further finished to a surface of a level that no roughness is observed by means of a differential interference microscope.
This invention relates to a composition for smooth polishing of the surface of a wafer widely used as a substrate for IC's, more specifically, to a polishing composition used in the final polishing process for smooth polishing of the surface of a wafer to remove roughness greater than 5 m.mu. present on the wafer surface.
2. Description of Prior Art
Heretofore, to polish the surface of a wafer, there have been proposed a variety of polishing agents.
For example, the specification of U.S. Pat. No. 3,170,273 discloses a silica sol containing 2 to 50% by weight of silica and a silica gel containing 2 to 100% of silica as polishing agents. The specification of U.S. Pat. No. 3,328,141 discloses that when these polishing agents are mixed with an alkaline compound to adjust the pH value to 10.5 to 12.5, the resulting polishing agents have increased polishing speeds. However, observation by a differential interference microscope, for example, or the like of the surface of wafers polished using these polishing agents has revealed roughness of 5 to 500 m.mu. therefore, these polishing agents are not satisfactory.
Japanese Patent Publication No. 53-9910/1978 discloses a polishing agent containing quartz, silicates, and hexafluorosilicates, and further containing a monohydric alcohol having 3 to 5 carbon atoms, and polyvinyl alcohol. However, the polished surface obtained has not been satisfactory, and the polishing agents containing these alcohols have problems in that they are difficult to store for a long time.
Furthermore, a composition comprising water-soluble carboxymethylene rubber or xanthane gum is disclosed in Japanese Patent Publication 61-14655/1986 (U.S. Pat. No. 4,260,396). However, this composition is to be used in a primary polishing process, at a polishing pressure of 3.9 pounds per square inch (=274 gr/cm.sup.2) and at a rate of 4.0 mils per hour (=1.7 microns per minute). When this composition is used at a low polishing pressure of about 100 g/cm.sup.2 those conditions which are applied to secondary polishing, to obtain a surface of such level that no irregularities can be observed, it requires a long polishing time and a long rinse polishing time for rinsing in flowing water after polishing. Moreover, the purpose for containing water-soluble carboxymethylene rubber or xanthane gum is to prevent the sedimentation of silica hydrogel. However, sedimentation of silica tends to take place even in the presence of the rubbers or gum, thus stirring or shaking is required before it is used as a polishing agent.
The specification of U.S. Pat. No. 4,462,188 discloses a polishing agent containing an aqueous colloidal silica gel or sol, an amine, a water-soluble quaternary ammonium salt or base. However, as can be seen from the examples, this polishing agent is too high in polishing speed, and is thus obviously to be used for the primary polishing process.
Furthermore, the specification of U.S. Pat. No. 4,664,679 discloses a method to obtain an aqueous dispersion of silicic acid anhydride used as a polishing agent. However, as can be seen from the examples, the polishing agent obtained from this method results in a large surface roughness (R-max) of 0.1 .mu.m, and is thus obviously to be used for the primary polishing process.
It is the primary object of the present invention to eliminate the difficulties of prior arts. Further, the objects are:
1. a polishing agent which gives a polished surface having no roughness detectable by a differential interference microscope,
2. a polishing agent which requires a short polishing time to obtain a smooth surface,
3. a polishing agent which requires a smaller amount of silica to obtain a smooth surface, and
4. a polishing agent which can be stably stored for a long time.